The unity gain buffer amplifier is a feedback amplifier that provides unity voltage gain at its output. The unity gain buffer is commonly used in many applications because of its superior characteristics. These characteristics include a relatively low output impedance and high input impedance, gain stability with respect to temperature, and an improved linearity.
This amplifier, as any feedback amplifier, suffers from a tendency to oscillate at higher frequencies. The simplest and most common method of compensating for oscillation is to reduce the bandwidth of the amplifier by adding capacitance. The capacitance introduces a dominant pole within the amplifier to force the phase shift of the gain to be less than -180.degree. and thus the phase margin to be greater than 0.degree. when the loop gain T is unity. This addition will satisfy the Nyquist criterion for stability. However, the trade-offs in amplifier performance are considerable. For one, the frequency capability of the amplifier is sacrificed. For another, the introduced phase margin causes peaking in the overall gain at the resonant (unity loop gain) frequency of the amplifier. The smaller the phase margin, the larger the gain peak becomes until the gain approaches infinity and oscillation occurs for a phase margin of 0.degree.. For a third reason, increasing the capacitance to increase the phase margin lowers the slew rate of the amplifier. The lower the slew rate, the longer the amplifier takes to respond to a large input signal.
The need to preserve stability is especially important in gain selectable amplifiers, where the current gain can be varied upon command. Such amplifiers may be stable at one level of current gain and yet be unstable at another level because of the change in feedback caused by changing resistance therein. The change in feedback within the amplifier affects the peaking of the gain at the resonant frequency.